Model and input conventions
Opening basis is far entry price minus near entry price. Closing basis uses the same subtraction at one common exit time. Net P&L equals matched underlying quantity multiplied by opening basis minus closing basis, less total costs. Equivalently, add the near long-leg price change and the far short-leg price change. Contracts must have matching base units and the same quote-currency linear payout convention.
Worked numerical example
Buy two underlying units of the near contract at 100 and sell two units of the far contract at 105. Exit the near at 102 and the far at 104. The near leg earns 4, the far leg earns 2, and a combined cost of 1 leaves net profit of 5. If both prices instead rise by the same ten points, the spread earns zero before costs and loses 1 after them.
Read the scenario table
The scenario grid changes near and far exit prices independently, then recalculates closing basis and total P&L. Rows with a narrowing far-minus-near basis benefit this specific long-near, short-far direction. A widening basis hurts it. The separate-leg metrics help identify a sign mistake or unequal contract multiplier before comparing the combined amount with a broker’s statement or a manually recorded trade.
Where the model stops
Different expiries can react differently to financing, positioning and settlement expectations. The calculator does not infer a guaranteed convergence date or return. Margin on each leg, execution timing, liquidation and financing of variation margin are omitted. Entering two prices from different exit times introduces another exposure that the synchronized scenario does not represent. An inverse or quanto contract cannot be substituted without changing its payoff conversion.